The present invention relates generally to antennas for receiving and transmitting radio frequency signals, and more specifically to such an antenna for receiving and transmitting radio frequency signals in multiple wireless communications frequency bands and with various radiation patterns.
With the expansive deployment of computer resources, it has become advantageous to connect computers to allow collaborative sharing of information. Conventionally, the connection is in the form of wired computer or data networks (generally referred to as local area networks or LAN""s) operating under various standard protocols, such as the Ethernet protocol. Users connected to the network can exchange data with other network users, irrespective of the physical distance between, the users. These networks, which have become ubiquitous among computer users, operate at fairly high speeds, up to about 1 Gbps, using relatively inexpensive hardware. However, LANs are limited to the physical, hard-wired infrastructure of the structure in which the users are located.
During recent years, the market for wireless communications of all types has enjoyed tremendous growth. Wireless technology allows people to exchange information using pagers, cellular telephones, and other wireless communication products. With the steady expansion of wireless communications, wireless concepts are now being applied to data networks, relieving the user of the need for a wired connection between the computer and the network.
The major motivation and benefit from wireless LANs is the user""s increased mobility. Untethered from conventional network connections, network users can access the LAN from wireless network access points strategically located within a structure or on a campus. Examples of the practical uses for wireless network access are limited only by the imagination of the application designer. Medical professionals can obtain not only patient records, but real-time vital signs and other reference data at the patient bedside without relying on reams of paper charts and physical paper. From anywhere on the factory floor, workers can access part and process specifications without impractical or impossible wired network connections. Wireless connections with real-time sensing allow a remote engineer to diagnose and maintain the health and welfare of manufacturing equipment. Warehouse inventories can be verified quickly and effectively with wireless scanners connected to the main inventory database. Frequently it is more economical to install a wireless LAN than to install a wired network in an existing structure. Wireless LANs offer the connectivity and the convenience of wired LANs without the need for expensive wiring or rewiring.
The Institute for Electrical and Electronics Engineers (IEEE) standard for wireless LANs (IEEE 802.11) sets forth two different wireless network configurations: ad-hoc and infrastructure. In the ad-hoc network, computers are brought together to form a network xe2x80x9con the fly.xe2x80x9d There is no structure to the network and there are no fixed network points. Typically, every node is able to communicate with every other node. The infrastructure wireless network uses fixed wireless network access points with which mobile nodes can communicate. These wireless network access points are typically bridged to landlines to allow users to access other networks and sites not on the wireless network.
The IEEE 802.11 standard governs both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which actually handles the transmission of data between nodes, can use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of either 1 Mbps or 2 Mbps, and calls for operation in the 2.4-2.4835 GHz frequency band (which is an unlicensed band for industrial, scientific, and medical (ISM) applications) and 300-428,000 GHz for IR transmission.
The MAC layer comprises a set of protocols that maintain order among the users accessing the network. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet for transmission over the network, it first listens to ensure no other node is transmitting. If the channel is clear, the node transmits the packet. Otherwise, the node chooses a random xe2x80x9cbackoff factorxe2x80x9d that determines the amount of time the node must wait until it is allowed to retry the transmission.
Several extensions of the IEEE 802.11 standard have been developed. The first, referred to as 802.11a, provides a data rate of up to 54 Mbps in the 5 GHz frequency band. The 802.11a standard requires an orthogonal frequency division multiplexing encoding scheme, rather than the frequency hopping and direct sequence spread schemes of 802.11. The 802.11b standard (also referred to as 802.11 high rate or Wi-Fi) provides a 11 Mbps transmission data rate, with a fallback to data rates of 5.5, 2 and 1 Mbps. The 802.11b scheme uses the 2.4 GHz frequency band, using direct sequence spread spectrum signalling. Thus 802.11b provides wireless functionality comparable to the Ethernet protocol. The newest standard, 802.11g provides for a data rate of 20+Mbps in the 2.4 GHz band. A primarily European wireless networking standard similar to the 802.11 standards, referred to as HyperLAN2, operates at 5.8 MHz.
Today, devices implementing either the 802.11a or 802.11b standard are available. The higher data rate of 802.11a devices can support bandwidth hungry applications, but the higher operating frequency limits the radio range of the transmitting and receiving units. Typically, 802.11a compliant radios can deliver 54 Mbps at distances of about 60 feet, which is far less than the 300 feet radio range over which the 802.11b systems can operate, albeit at lower data rates. Thus 802.11a installations require a larger number of media access points from which users link into the network.
Recognizing the advantages and disadvantages of the two standards, the current market trend is to develop dual mode communications devices that take advantage of the 802.11a protocol, but provide for a fall back mode at the lower data rates of the 802.11b systems when an adequate communications link cannot be established under the 802.11a standard. Software processors in the receiving and transmitting units can accommodate operation under either standard.
According to the prior art, such dual-mode devices use either a single broadband antenna or multiple single-band antennas. No effective multiple or dual band antennas are available. The known broadband antennas capable of operating in both the 802.11a and 802.11b frequency bands represent poor choices due to their high gain at frequencies outside the 802.11a and 802.11b operational bands. The wide bandwidth allows extraneous noise and interfering signals to enter the transmitter/receiver, degrading the signal-to-noise ratio and limiting the data rate. Thus the wide bandwidth imposes more restrictive requirements on the radio frequency filters. Use of multiple single-band antennas requires complex and space-hungry feed and switching structures for multiple band operation, as each antenna requires a dedicated feed network. Since it is generally required to fit the antenna into a small space within the communications device, space it as a premium and thus multiple single-band antennas are not preferred.
The present invention comprises a plurality of layers in stacked relation, including a lower conductive plate, a middle conductive plate, an upper conductive plate, a lower dielectric layer disposed between the lower conductive plate and the middle conductive plate and an upper dielectric layer disposed between the middle conductive plate and the upper conductive plate. The antenna further comprises a first ground conductor extending between and electrically connected to the upper conductive plate and the lower conductive plate, a second ground conductor extending between and electrically connected to the middle conductive plate and the lower conductive plate, and a signal feed conductor connected to the upper conductive plate. The antenna advantageously presents a resonance condition in several frequency bands.